The present invention generally relates to forging equipment and processes, including those used in the production of large forgings from metal powders. More particularly, this invention relates to a forging die equipped with radial segments that reduce the incidence of cracking during forging of powder metallurgy billets by promoting radial growth during forging.
Rotor components for power generation turbines have typically been formed of iron and nickel-based alloys with low alloy content, i.e., three or four primary elements, which permit their melting and processing with relative ease and minimal chemical or microstructural segregation. Recently, wheels, spacers, and other rotor components of more advanced land-based gas turbine engines used in the power-generating industry, such as the H and FB class gas turbines of the assignee of this invention, have been formed from high strength alloys such as gamma double-prime (γ″) precipitation-strengthened nickel-based superalloys, including Alloy 718 and Alloy 706. Typically processing of these components include forming ingots by triple-melting (vacuum induction melting (VIM)/electroslag remelting (ESR)/vacuum arc remelting (VAR)) to have very large diameters (e.g., up to about 90 cm), which are then billetized and forged. In contrast, rotor components for aircraft gas turbine engines are often formed by powder metallurgy (PM) processes, which are known to provide a good balance of creep, tensile and fatigue crack growth properties to meet the performance requirements of aircraft gas turbine engines. Powder metal components are typically produced by consolidating metal powders in some form, such as extrusion consolidation, then isothermally or hot die forging the consolidated material to the desired outline.
The use of powder metallurgy processes to produce large forgings suitable for rotor components of power-generating gas turbine engines provides the capability of producing more near-net-shape forgings, thereby reducing material losses. As more complex alloys such as Alloy 718 and beyond become preferred and the size of forgings continues to increase, the concerns of chemical and microstructure segregation, high material losses associated with converting large grained ingots to finish forgings, and limited industry capacity to process large, high strength forgings make the higher base cost PM alloys potentially more cost effective. However, problems encountered when forging powder metallurgy billets include high frictional forces that develop at the die-billet interface and impede free radial growth of the billet, resulting in cracks in the forging. These cracks, believed to be driven by tangential stresses, have been observed to be regularly spaced and in the radial direction at the Poisson-induced bugle in the forging during the upset process. Proposed solutions to this problem, including varying the forging die temperature, upset levels, and forging strain rates, have achieved only limited success.